Poriferous microtablet of anatase TiO2 growth on an ITO surface for high-efficiency dye-sensitized solar cells
Introduction
The performance of dye-sensitized solar cell (DSSC) devices is determined by the effectiveness of the photo-physicochemical processes that occur on the surface of the wide-band-gap semiconductor materials (SCs) incorporated into the structure. Such a surface reaction process might be enhanced if the SC structure provides a wide, active surface area for both reaction and dye loading. Recently, there have been significant attempts to synthesize SC materials that have a large active surface area structure. An extensive range of morphologies have been proposed, such as nanowires, [1] nanograss, [2] nanorods, [3], [4] nanotubes, [5], [6] and porous nanotubes [7]. To date, TiO2 nanotubes appear to be the structure that provides the largest surface area for both dye chemisorption and photoactivity [4], [5], [6], [8], [9], [10], [11], [12], [13]. Nevertheless, despite its intrinsically large surface area, realizing the enhanced performance expected from efficient surface photo-physicochemical activity on this structure is predicted to be difficult. In fact, depending on the nanotube diameter, the possibility of both dye and electrolyte contact with the inner wall of the nanotube (a unique feature for surface area superiority) decreases with a reduction in the nanotube diameter [14]. This is the result of an increase in the inter-molecular attractive force between the liquid and the nanotube end that will prohibit the electrolyte to enter into and to make contact with the inner side surface of the nanotube. Therefore, there is an effort to discover a nanostructure that simultaneously has a morphology exhibiting a large surface area and a structure that facilitate effective surface photo-physicochemical processes.
For the first time, we report the synthesis of poriferous TiO2 microtablets (PTMs) directly onto a substrate surface using a simple liquid-phase deposition method as an alternative for a TiO2 structure with a large active surface area. By simply controlling the concentration of the growth solution, which is composed of ammonium hexafluorotitanate and boric acid, a controlled density of PTM on the substrate surface can be obtained. The PTM surface or “skin” is decorated by small nanowires of up to 200 nm in length, and the bulk (body) structure is constructed of a brick-like assembly of nanocuboids. With such a hairy surface and a high porosity bulk structure, it is possible to obtain an effective photo-physicochemical process and high dye loading for the structure. Typically, the nanowires that decorate the surface are arranged with reasonably wide spacing, which allows efficient contact with the liquid phase. With its unique surface planes (particularly those with high surface energy), the nanowires may further provide efficient catalytic reactions with the redox species in the photoelectrochemical (PEC) device [15], [16]. In addition to this special feature, the nanowire ends may also contain a high density of electrons, which enhances redox reactions in the system [17]. A DSSC device has been fabricated using the PTM synthesized. A light-to-current conversion efficiency as high as 3.0% has been obtained under simulated AM 1.5 illumination (100 mW/m2). The PTMs are predicted to be highly useful for solar cells and for photocatalytic applications because of their simple preparation procedure and their unique morphology, which may facilitate enhanced-carrier transport [18] to the external circuit or reduce recombination losses in the device [16], [19].
Section snippets
Poriferous TiO2 microtablet (PTM) preparation and characterization
The poriferous TiO2 microtablets (PTM) were prepared using a liquid-phase deposition (LPD) method. In a typical procedure, a clean ITO substrate (purchased from CBC Ing. Co., Ltd., Japan, with a sheet resistance of ca. ~10 Ω/square), washed using a standard cleaning procedure in acetone and ethanol under ultrasonication, was immersed into the growth solution containing 5 mL of 0.5 M (NH4)2TiF6 (Sigma Aldrich) and 5 mL of 1.0 M H3BO3 (WAKO Company, Japan). The substrate was positioned vertically in
Poriferous TiO2 microtablet (PTM) characterization
In the normal liquid-phase deposition (LPD) method, a continuous TiO2 film, usually with a cracked-structure, is obtained on the substrate surface after an immersion process in a growth solution that contains equimolar amounts of ammonium hexafluorotitanate and boric acid. This process is generally carried out at ambient conditions and followed by thermal annealing for phase purification [20], [21]. Here, unlike in the normal LPD method, after altering only, the precursor concentration,
Conclusion
The synthesis of poriferous TiO2 microtablets (PTM) with surfaces decorated by high-density nanowire skins, which are grown onto an ITO surface, has been demonstrated. The PTM are characterized by a square morphology with an edge-length and a thickness of approximately 10 and 5 μm, respectively. They are constructed from two different structures, a brick-like assembly of nanocuboids and a nanowire skin in the interior and the exterior, respectively. Their formation is thought to be a successful
Acknowledgments
The authors are grateful for the financial support received from the Ministry of Higher Education of Malaysia under the research fundamental FRGS/1/2012/SG02/UKM/02/3, the Ministry of Science, Technology and Innovation under the research grant of 03-01-02-SF0836, and the Universiti Kebangsaan Malaysia under the DIP-2012-16 and GUP-2013-030 projects.
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